1. Field of the Invention
This invention relates to an ultrasonic wave generator, and more particularly to such generator having a vibratory member for producing large-amplitude ultrasonic waves.
2. Prior Art
Generally, an ultrasonic wave generator in common use has heretofore been of the type essentially comprising an ultrasonic electromechanical transducer, an ultrasonic horn and an ultrasonic frequency oscillator, or that comprising a cylindrically shaped ultrasonic electromechanical transducer and an ultrasonic frequency oscillator.
In the former type using the ultrasonic horn, the arrangement is such that electrical vibrations from the ultrasonic frequency oscillator be transformed into mechanical vibrations (longitudinal vibrations) by the ultrasonic electromechanical transducer of either the piezo-electrical type incorporating piezoelectric elements such for example as PZT, or the magnetostrictive type incorporating magnetostrictive elements such as ferrite; and the thus produced mechanical vibrations be transmitted to the ultrasonic horn connected to the output end of the electromechanical transducer, so that the vibrations which have been increased in amplitude by the same horn are released in the form of ultrasonic waves from the mechanical vibration output end of the horn.
As is commonly known in this arrangement, the ultrasonic horn serves the purpose of amplitude magnification, because the amplitude of any ultrasonic wave obtainable with the ordinary electromechanical transducer is significantly small, such for examples as 4.mu. (peak to peak) at maximum with a 40 KHz-transducer. With respect to that magnification, also, its value is determined by a function of the area ratio S.sub.2 /S.sub.1 in the horn, wherein S.sub.2 is the cross-sectional area at the mechanical vibration input end and S.sub.1 that at the mechanical vibration output end, or the amplifying power will be greater as the area ratio increases. To describe more specifically, the amplifying power is determinable in such a manner that with a "stepped" horn having step-like varied cross-sections, it is determined by the value of S.sub.2 /S.sub.1 ; with an "exponential" horn exponentially tapered, it is by the square root of S.sub.2 /S.sub.1 ; and with a "conical" horn linearly tapered, it is determined by a function of the square root of S.sub.2 /S.sub.1.
On the other hand, with respect to the geometrical requirements of the ultrasonic horn, it is commonly necessary that for attainment of an efficient amplification of ultrasonic vibrational amplitudes, the diameter of the horn at the input end be not exceeding 1/4 of a wavelength .lambda. of the ultrasonic wave, while its length being chosen so as to enable the horn to effect longitudinal resonant vibrations at the same frequencies as those provided by the transducer.
It follows, therefore, that the maximum amplification power of enlarging amplitudes of ultrasonic vibrations by the horn will be determined of its value as obtainable with the maximum diameter of the input end, i.e., .lambda./4 and the minimized area of the output end.
However, in the above arrangement of ultrasonic wave generating system, the problem lies in that when providing the horn with such higher rates of amplification, the mechanical vibration output end of the horn is much reduced in area, as for example about 0.9 cm in diameter in relation to the input end of 3 cm dia., under the conditions of 40 KHz and ten amplifications. This has thus formed a known drawback of the ordinary ultrasonic wave generator that in practice, ultrasonic vibrations of large amplitudes are only applicable to rather a limited working zone.
Further, as regards another type using the cylindrically shaped ultrasonic electromechanical transducer wherein piezoelectrical elements such as PZT are in the cylindrical form, it is arranged that electric oscillations are transformed by the cylindrical transducer into mechanical vibrations. This arrangement is admittedly advantageous in respect of an enlarged area of vibration because of its cylindrical shape. However, as the entire surface of the cylinder effects a uniform ultrasonic vibrational motion in the radial direction, it is not attainable to amplify the vibration by means of an ultrasonic horn in such a manner as described with respect to the previous arrangement of the prior art. Hence, this arrangement also has involved a drawback that since the maximum amplitude of vibration obtainable is specifically limited, as for example below 4.mu. (peak to peak) with a 40 KHz-transducer, the arrangement is by no means suited to the purpose of a large-amplitude ultrasonic wave generator.